HomeMy WebLinkAboutFORT COLLINS NISSAN-KIA - PDP210017 - DOCUMENT MARKUPS - ROUND 4 - DRAINAGE REPORTPRELIMINARY DRAINAGE REPORT
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Fossil Creek Subdivision
Fort Collins, CO
Prepared for:
Fort Collins Nissan-Kia
5811 S College Ave
Fort Collins, CO 80526
Date:
April 13, 2022
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April 13, 2022
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE: Fort Collins Nissan-Kia
Fort Collins, Colorado
Project Number: U21004
Dear Staff:
United Civil Design Group, LLC. is pleased to submit this Preliminary Drainage Report for the Fort Collins Nissan-Kia site in
Fort Collins, Colorado. In general, this report serves to document the stormwater impacts associated with the proposed
improvements related to the existing site.
We understand that review by the City of Fort Collins is to assure general compliance with standardized criteria contained in
the Fort Collins Stormwater Criteria Manual (FCSCM). This report was prepared in compliance with technical criteria set forth
in the Fort Collins Stormwater Criteria Manual.
If you should have any questions or comments as you review this report, please feel free to contact us at your convenience.
Sincerely,
United Civil Design Group
Colton Beck, PE Kevin Brazelton, PE
Project Engineer Principal
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TABLE OF CONTENTS
I. General Location and Description ...................................................................................................................................... 1
A. General Location & Existing Site Information ............................................................................................. 1
B. Description of Property ............................................................................................................................... 2
C. Project Description ..................................................................................................................................... 2
D. Floodplains .................................................................................................................................................. 3
II. Drainage Basins and Sub-Basins ......................................................................................................................................... 3
A. Major Basin Description .............................................................................................................................. 3
B. Sub-Basin Description ................................................................................................................................. 3
III. Drainage Design Criteria ..................................................................................................................................................... 3
A. Regulations ................................................................................................................................................. 3
B. Directly Connected Impervious Area (DCIA) ............................................................................................... 4
C. Hydrological Criteria ................................................................................................................................... 4
D. Hydraulic Criteria ........................................................................................................................................ 4
E. Modifications of Criteria ............................................................................................................................. 5
IV. Drainage Facility Design ..................................................................................................................................................... 5
A. General Concept ......................................................................................................................................... 5
B. Specific Details ............................................................................................................................................ 5
V. Erosion Control ................................................................................................................................................................. 14
VI. Conclusions ...................................................................................................................................................................... 15
A. Compliance with Standards ...................................................................................................................... 15
B. Drainage Concept...................................................................................................................................... 15
C. Stormwater Quality................................................................................................................................... 15
VII. References .................................................................................................................................................................... 15
APPENDICES
APPENDIX A – Hydrology Calculations
APPENDIX B – Hydraulic Calculations
B.1 – Low Impact Development Calculations
B.2 – Water Quality Calculations
B.3 – Detention Pond and Orifice Calculations
B.4 – Channel Capacity Calculations
B.5 – Inlet Sizing Calculations (Reserved for Final Submittal)
B.6 – Storm Pipe Calculations (Reserved for Final Submittal)
B.7 – Curb Channel Calculations (Reserved for Final Submittal)
APPENDIX C – Referenced Materials
APPENDIX D – Drainage Exhibits
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I.GENERAL LOCATION AND DESCRIPTION
A.GENERAL LOCATION &EXISTING SITE INFORMATION
The Fort Collins Nissan-Kia site (referred herein as “the site”) is located within the Fossil Creek Subdivision, situated in the
northeast quarter of Section 11, T6N, R69W of the 6th P.M., City of Fort Collins, Larimer County, Colorado. The property,
consisting of approximately 17.0 acres, is located immediately west of S College Ave. The site is split into two separate areas
divided by Crestridge St. South of Crestridge St. exists a car dealership to be redesigned and redeveloped. This portion of the
property is bounded by Crestridge St to the north, S College Ave to the east, a school facility and residential land to the west,
and a commercial property to the south. On-site stormwater drains off-site undetained to either S College Ave, Crestridge St,
or off-site to the west.
The northern, undeveloped portion of the site is bounded by Crestridge St to the south, S College Ave to the east, and
undeveloped land to the north and west. Stormwater on this portion of the site largely drains the northwest via overland
flow to Fossil Creek. A portion of the site adjacently north of Crestridge St drains to the south via overland flow. All on-site
runoff drains downstream within the City of Fort Collins Fossil Creek master drainage basin.
Storm sewer infrastructure exists on-site south of Crestridge St and drains to the off-site to the east. This storm pipe exists as
an irrigation bypass system. A City of Fort Collins Floodway and an Erosion Buffer are established with Fossil Creek and Lang
Gulch. Also, existing on-site is an underground irrigation pipe owned by North Louden Ditch Company. This irrigation pipe
drains from south to north through the site, ultimately to infrastructure that drains across S College Ave to the east. The on-
site portion of this ditch lateral is planned to be relocated; however, this relocation is unrelated to the drainage design of the
site.
An existing residential subdivision and small commercial property are constructed to drain runoff to Venus Ave, conveyed
through an existing wetland area west of Venus Ave to Lang Gulch and ultimately to Fossil Creek.
FIGURE 1:SITE VICINITY MAP
Smokey St S College AveBueno Dr Venus AveCrestridge St
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B.DESCRIPTION OF PROPERTY
The northern portion of the project site currently exists largely as grass/weed landscape. The southern portion of the site
includes concrete and asphalt pavement, a commercial building, and gravel landscaping. In its historic condition, the site
primarily sheet flows in two general directions: north and south, all conveyed to Fossil Creek.
Below are summaries of key components of the site in its existing conditions.
Land Use - The site’s current land use is commercial.
Ground Cover - The northern portion of the site exists as grass/weed landscape. The weed/grass cover is good (ie.,
heavy or dense cover with nearly all ground surfaces protected by vegetation). The southern portion of the site is
fully developed as consists largely of impervious hardscaping.
Existing Topography – The northern, undeveloped portion of the site is bounded by Crestridge St to the south, S
College Ave to the east, and undeveloped land to the north and west. Stormwater on this portion of the site largely
drains the northwest via overland flow to Fossil Creek. A portion of the site adjacently north of Crestridge St drains
to the south via overland flow. All on-site runoff drains downstream within with the City of Fort Collins Fossil Creek
master drainage basin.
Grades – In general, the northern portion of the site slopes northwesterly at approximately 1.0% to 15.0%; the
southern portion of the site is sloped in several directions at approximately 1.0% to 3.0%
Soil Type – The USDA’s Web Soil Survey shows that the site consists of “Type B”, “Type C”, and “Type D” soils. The
Web Soil Survey indicates the site is comprised of Kim Loam (5.0 to 9.0% slopes), Kim Thedalund Loams (3.0 to 15.0%
slopes), Midway Clay Loam (5.0% to 25.0% slopes) and Thedalund Loam (3.0% to 9.0% slopes). The on-site soils
provide moderate infiltration and are suitable for development.
Utilities – The following utilities exist adjacent to the site in Crestridge St, College Avenue and Venus Drive: potable
water, sanitary sewer, natural gas, electric and telecommunications.
Irrigation Facilities – An existing on-site underground irrigation pipe owned by North Louden Ditch Company. This
irrigation pipe drains from south to north through the site, ultimately to infrastructure that drains across S College
Ave to the east.
Drainage Features and Storm Sewer – Storm sewer infrastructure exists on-site south of Crestridge St. North of the
site exists the Fossil Creek. For development purposes, a City of Fort Collins Floodway and an Erosion Buffer are
established with Fossil Creek. The western side of the site drains via sheet flow to the Lang Gulch. This portion of the
Lang Gulch exists immediately upstream of Fossil Creek and is accounted for within the City of Fort Collins Floodway
boundary.
C.PROJECT DESCRIPTION
The proposed Fort Collins Nissan-Kia site improvements are limited to approximately 19.0 acres of disturbance. South of
Crestridge St, the proposed site improvements consist of the demolition of the existing car dealership building and associated
paving, and ultimately the construction of a new building with associated drive lanes and inventory parking lot. North of
Crestridge St, site improvements include the extension of Venus Ave, and the addition of a car dealership building and
associated drive lanes and inventory parking lot. Designed with the proposed commercial buildings and paving include
underground utilities, underground water quality, and underground detention. On-site stormwater south of Crestridge Dr is
designed to be conveyed via storm sewer to proposed drainage facilities west of Venus Ave.
In addition to an existing school facility adjacent to the site at the intersection of the Venus Ave and Crestridge St, the existing
Skyview Subdivision to the south is associated with the site in regard to stormwater runoff. In its existing state, this
development along with the school facility drains approximately 3.3-acres to an existing on-site wetland area. With a
proposed relocation of the mentioned wetlands, runoff is now designed to be conveyed to a proposed on-site detention
facility.
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D.FLOODPLAINS
The proposed development is not in a FEMA
designated floodplain. According to the FEMA
Flood Insurance Rate Map Panel (08069C1000F),
the site is zoned in an area of minimal flood
hazard. The FEMA FIRM Map is included in the
appendix.
West and Northwest of the site improvements
exists a “High Risk – Floodway” boundary and
erosion buffer along Lang Gulch and Fossil
Creek, established by the City of Fort Collins. The
Nissan-Kia property boundaries exist within the
City of Fort Collins Floodway boundary and
erosion buffer; however, the bounds of grading
construction is to be completed outside of this
boundary. The City of Fort Collins Floodway
boundary is provided on the Drainage Plan (see
Appendix D) and a City of Fort Collins floodplain
map is included in Appendix C. Should
construction be required within the floodway, a
Floodplain Use Permit with a No-Rise
Certification will be required and all work will be
required to be in compliance with Chapter 10 of
the City Municipal Code. If work within the
erosion buffer is required, a erosion buffer waiver
will be required.
II.DRAINAGE BASINS AND SUB-BASINS
A.MAJOR BASIN DESCRIPTION
The proposed development is located within the Fossil Creek Basin and will adhere to the Fossil Creek Basin Master Drainage
Plan. In conformance with master plan, the site is designed to release on-site runoff at a rate not to exceed 0.2 cfs/acre.
B.SUB-BASIN DESCRIPTION
Historically the southern portion of the property has been utilized for commercial purposes. The site is designed to operate
in a similar fashion (i.e., commercial), though with significant site improvements. Despite these significant improvements, the
proposed site is largely designed to conform to existing drainage patterns – downstream within the Fossil Creek Basin. In its
existing condition, stormwater on the northern portion of the site is largely conveyed the north to Fossil Creek. The southern
portion of the property drains in a multitude of directions: west to a neighboring property, east to US 287, and north to
Crestridge St. In its improved condition, on-site runoff is intended to drain to multiple downstream underground water quality
and detention facilities by way of sheet flow, swales, pans, inlets, and storm sewer.
III.DRAINAGE DESIGN CRITERIA
A.REGULATIONS
The design criteria for this study are directly from the City of Fort Collins Storm Drainage Design Criteria and Construction
Standards Manual and the Urban Storm Drainage Criteria Manual’s (referred to herein as USDCM) Volumes 2, and 3, used
for supplement only.
FIGURE 2:FLOODPLAIN MAP
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B.DIRECTLY CONNECTED IMPERVIOUS AREA (DCIA)
The City drainage criteria has also adopted the “Four Step Process” that is recommended in Volume 3 of the USDCM in
selecting structural BMPs for the redeveloping urban areas. The following portions of this summary describe each step and
how it has been utilized for this project:
Step 1 – Employ Runoff Reduction Practices
The objective of this step is to reduce runoff peaks and volumes and to employ the technique of “minimizing directly
connected impervious areas” (MDCIA). This project accomplishes this by:
Routing the roof and pavement flows through a bioretention facility, underground water quality treatment/infiltration
facilities and vegetated swales to increase time of concentration, promote infiltration and provide water quality.
Step 2 – Provide Water Quality Capture Volume (WQCV)
The objective of providing WQCV is to reduce the sediment load and other pollutants that exit the site. For this project WQCV
is provided within underground treatment facilities, a bioretention pond and an extended detention pond.
Step 3 – Stabilize Drainageways
The site exists adjacent to lang Gulch and Fossil Creek. Although the project will not be directly connecting or stabilizing either
drainageway, the project will enhance the stabilization of the existing drainageways by reducing peak flows to the
drainageways via extended detention; and in the case of Lang Gulch, detaining a portion of an offsite basin that currently free
releases to the drainageway. In addition, this project will pay stormwater development and stormwater utility fees which
the City uses, in part, to maintain the stability of the City drainageway systems.
Step 4 – Consider Need for Site Specific and Source Control BMPs
Site specific and source control BMPs are generally considered for large industrial and commercial sites. The redevelopment
of the existing site will include multiple site specific and source controls, including:
Covered storage areas for vehicles and materials.
Dedicated vehicle maintenance areas within each build with appropriate internal waste collection systems. Sand/oil
separators are provided with the utility plans.
Dedicated maintenance personnel providing landscape maintenance and snow and ice management.
Bioretention water quality control ponds.
C.HYDROLOGICAL CRITERIA
City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, provided by Table 3.4-1 of the Fort Collins Stormwater
Criteria Manual, are utilized for all hydrologic computations related to the site in its existing/historic and proposed conditions.
Since this site is relatively small and does not have complex drainage basins, the peak flow rates for design points have been
calculated based on the Rational Method as described in the USDCM and the City of Fort Collins Stormwater Criteria Manual
(FCSCM) with storm duration set equal to the time of concentration for each sub-basin. This method was used to analyze the
developed runoff from the 2-year (minor) and the 100-year (major) storm events. The Rational Method is widely accepted
for drainage design involving small drainage areas (less than 160 acres) and short time of concentrations. Runoff coefficients
are assumed based on impervious area and are given in the Appendices.
D.HYDRAULIC CRITERIA
The developed site will convey runoff to the proposed pond or underground treatment facility via swales, concrete channels,
and pipes. The City of Fort Collins Stormwater Criteria Manual (FCSCM) and USDCM are referenced for all hydraulic
calculations. In addition, the following computer programs are utilized:
Storm Sewer Extension for AutoCAD Civil3D
Hydraflow Express Extension for AutoCAD Civil3D
UD-Inlet by UDFCD
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Drainage conveyance facility capacities proposed with the development project, including storm sewer, swales, and inlet
capacities, are designed in accordance with criteria outlined in the FCSCM and/or the Urban Drainage and Flood Control
District’s Urban Storm Drainage Criteria Manual (UDFCD) used for supplement purposes only.
E.MODIFICATIONS OF CRITERIA
The conveyance of a considerable amount of off-site stormwater, in addition to hardships related to on-site detention,
present unique challenges concerning the release of stormwater in the site’s improved condition. To meet The Fossil Creek
Basin release criterion of 0.2 cfs/acre, while decreasing the overall release within the Fossil Creek Basin relative to existing
conditions, the solution presented in this report includes the “swapping” of detention throughout the site. We believe
manipulating the release rate to Lang Gulch and Fossil Creek outside the bounds of 0.2 cfs/acre for specific on-site drainage
basins, while overdetaining off-site stormwater, is beneficial to downstream features. Despite this variance, the overall
release from the site meets the 0.2 cfs/acre criterion, and the overall release from the site is significantly reduced.
IV.DRAINAGE FACILITY DESIGN
A.GENERAL CONCEPT
Developed runoff is designed to be conveyed in a safe and effective manner via swales, concrete channels, and storm sewer
systems/inlets. Stormwater is designed to be released within the Fossil Creek Basin. Runoff that drains off-site to the west
(south of Crestridge St) is heavily reduced with the proposed site. Runoff from the site in its improved condition is primarily
conveyed to underground water quality and detention facilities. Due to grading constraints, not all on-site runoff is fully
detained - these areas are considered in the overall release rate from the site.
B.SPECIFIC DETAILS
Hydrology
As previously stated, hydrology from the developed site is designed to comply with criteria set forth in the City of Fort Collins
Stormwater Criteria Manual. Referenced tables, charts, formulas, etc. are included in the appendices. The area, time of
concentration, and runoff of each proposed sub-basin is summarized in Appendix A. The following information outlines the
basin characteristics and drainage patterns for each basin.
Existing Basins
The following basins provide proposed drainage delineations for the site in its existing condition. Refer to Appendix A for
hydrology computations and Appendix D for visual representation of existing basins.
Basin EX1
Basin EX1 (refer to attached Existing Drainage Map) represents runoff associated with CDOT Right-of-Way in College Ave.
This basin consists of concrete paving, asphalt paving, gravel, and landscaping. Runoff within this basin is conveyed north
along College Ave and ultimately to Fossil Creek.
Basin EX2
Basin EX2 represents on-site runoff that drains to College Ave. This basin consists of concrete paving, asphalt paving, gravel,
and landscaping. Runoff within this basin is conveyed College Ave (i.e., Basin EX1) via sheet flow and concrete curb.
Basin EX3
Basin EX3 represents runoff associated with CDOT Right-of-Way in College Ave. This basin consists of concrete paving, asphalt
paving, gravel, and landscaping. Runoff within this basin is conveyed south along College Ave within the Fossil Creek Basin.
Basin EX4
Basin EX4 represents on-site runoff that drains to College Ave. This basin consists of concrete paving, asphalt paving, gravel,
and landscaping. Runoff within this basin is conveyed to College Ave (i.e., Basin EX3) via sheet flow and concrete curb.
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Basin EX5
Basin EX5 represents on-site runoff that drains to Fossil Creek. This basin is to remain undeveloped, consisting of landscaped
area. Runoff within this basin is conveyed to the northwest, designed to drain to off-site to a culvert system associated with
Fossil Creek.
Basin EX6
Basin EX6 represents on-site runoff that drains to the Lang Gulch, and ultimately Fossil Creek. A portion of this basin is
developed and is designed to drain to existing wetlands located around the southwestern region of the site; other
undeveloped areas drain directly to the Lang Gulch via sheet flow. This basin consists of concrete paving, asphalt paving,
gravel, and landscaping.
Basin OS1
Basin OS1 is a drainage basin associated with and adjacent commercial property and the Skyview Subdivision, located
immediately south of the site. This basin is fully developed and drains on-site via sheet flow along Venus Ave. This basin
accounts for approximately 3.3-acres of off-site drainage.
Basin OS2
Basin OS2 represents off-site runoff that drains to Fossil Creek. This basin is to remain undeveloped, consisting of landscaped
area. Similar to Basin EX5, runoff within this basin is conveyed to the northwest to a 12” culvert system associated with Fossil
Creek.
Proposed Basins
The following basins provide proposed drainage delineations for the site in its improved condition. Refer to Appendix A for
hydrology computations and Appendix B for calculations related to Detention, Water Quality, Low Impact Development, and
other hydraulic features.
Basin A
Sub-basins A1-A16 (see Drainage Plans attached) are generally located around the southern region of the site. These basins
consist of roofs, asphalt and concrete paving, and landscaping. Runoff within these Basins is conveyed to a Bioretention Pond
(Pond A2) via sheet flow, curb, inlets, and storm sewer. Runoff conveyed to the Bioretention Pond is further discharged
southwest to a detention facility (Pond A1). Runoff from Basin A is released to Lang Gulch and ultimately to Fossil Creek.
Basin B
Sub-basins B1-B7 represent on-site drainage basins generally located around the central region of the site. These basins
consist of roofs, asphalt and concrete paving, and landscaping. Runoff within these basins is conveyed to Bioretention Pond
A3 and Detention Pond A1. Runoff from Basin B is released to Lang Gulch via Pond A1 and ultimately to Fossil Creek.
Basin C
Sub-basins C1-C4 represent on-site drainage basins generally located around the northern region of the site. These basins
consist of roofs, asphalt and concrete paving, and landscaping. Runoff within Basins C3 and C4 is conveyed to a proposed
underground water quality and detention pond facility (Pond C) via inlets and storm sewer. Basin C1 and C2 (i.e., Venus Ave)
is conveyed to storm sewer in Venus Ave, and treated within underground water quality chambers prior to being released
downstream to Fossil Creek.
Basin D
Basin D1 represents CDOT Right-of-Way in College Ave at the Southeast corner of the site. In accordance with established
drainage patterns, this area of College Ave drains south along College Ave, ultimately to Fossil Creek. The basin consists of
concrete pavement, asphalt pavement, and landscaping.
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Basin E
Basin E1, similar to Basins D1, represents CDOT Right-of-Way in College Ave; however, this portion of College Ave drains to
the north. In accordance with established drainage patterns, this area of College Ave drains north along College Ave,
ultimately to Fossil Creek. The basin consists of concrete pavement, asphalt pavement, and landscaping.
Basin OS1
Basin OS1 is a drainage basin associated with and adjacent commercial property and the Skyview Subdivision, located
immediately south of the site. This basin is fully developed and drains on-site via sheet flow along Venus Ave. This basin
accounts for approximately 3.3-acres of off-site drainage. In its improved condition, Venus Ave is designed to convey Basin
OS1 to a proposed detention facility (i.e., Pond A1) via storm sewer infrastructure.
Basin OS2
Basin OS2 represents off-site runoff that drains to Fossil Creek. This basin is to remain similar to its existing condition, though
the basin is designed to drain to an improved culvert system to be provided at the downstream Fossil Creek Trail.
Stormwater Quality
I.Bioretention
Bioretention is designed to remove sediment and pollution from stormwater runoff via infiltration through an
engineered sand media. This slows the conveyance rate of storm water runoff, reduces runoff and provides a flow
path across a vegetated surface. Bioretention ponds are proposed with the site improvements and are utilized to
convey and treat stormwater prior to release to downstream proposed detention facilities and ultimately Fossil
Creek.
Bioretention Pond A2 is designed to provide approximately 11,600-sf of bioretention area. Provided 12-inches of
depth, the bioretention pond provides an approximate volume of 12,430-cf – this volume exceeds the required
WQCV for Basin A2 and Basins A4-16 (7,220-cf). Ponding above the initial 12-inches of pond depth is intended for
head purposes to discharge the 100-year event through the outlet structure. Runoff is ultimately conveyed to the
adjacent Detention Pond A1. Below is a summary of Bioretention Pond A2:
Pond Bottom of
Pond Elevation
Bioretention
WSEL
Bioretention Volume
Required
(cf)
Bioretention Volume
Provided
(cf)
Pond A2 5009.00 5010.00 7,220 12,430
Bioretention Pond A3 is designed to provide about 3,940-sf of bioretention area. Provided 12-inches of depth, the
bioretention pond provided an approximate volume of 3,630-cf – this volume exceeds the required WQCV for Basins
B2-B7 (2,854-cf). Ponding above the initial 12-inches of pond depth is intended for head purposes to discharge the
100-year event over an overflow weir. Below is a summary of the bioretention pond, recognized as Pond A3:
Pond Bottom of
Pond Elevation
Bioretention
WSEL
Bioretention Volume
Required
(cf)
Bioretention Volume
Provided
(cf)
Pond A3 5006.40 5007.40 2,854 3,630
II.Underground Water Quality
An underground water quality facility is designed to divert storm water runoff during the water quality event and
treat the flows through infiltration prior to being collected within an underdrain system. This provides for the
removal of sediment and other pollutants from the runoff while also promoting infiltration. Underground water
quality chambers are designed in two separate locations on-site to collect and treat storm water runoff within Basin
C prior to being released to downstream storm sewer associated with Basin C.
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The first underground water quality system, designated as “Pond C1”, is designed with a volume equivalent to the
required WQCV (i.e., 1,857-cf). The insulator row designed with the underground detention system will exceed the
volume required for water quality purposes; additional volume will be accounted for with the rest of the chambers
designed for detention purposes. Below is a summary table for water quality related to Basins C3 and C4. Refer to
Appendix B for additional information.
Pond
WQCV Required
(cf)
WQCV Provided
(cf)
Pond C1 1,857 1,857
The other underground water quality system, designated as “Pond C2”, is designed with a volume of 1,567-cf which
exceeds the minimum design requirements for the area (856-cf). Below is a summary table for water quality related
to Basins C1 and C2. Refer to Appendix B for additional information.
Pond
WQCV Required
(cf)
WQCV Provided
(cf)
Pond C2 856 1,567
Water quality treatment for Basin C1 is ultimately the responsibility of the adjacent development; however, due to
the build out of the Venus Ave roadway, water quality treatment is required with the installation of the northwestern
half of Venus Ave. The underground chambers are designed to be a permanent facility for the proposed Basins C1
and C2 only. Future off-site development of the neighboring property to the northwest is to provide a separate
private water quality system for the basin west of Venus Avenue.
Downstream of the Pond C1 and Pond C2 outfall is a 12” culvert under the Fossil Creek Trail. In its existing condition,
approximately 4.7-cfs is conveyed through the 12” culvert prior to overtopping the Fossil Creek Trail – this culvert is
not sufficient to convey the historic 10-year event (i.e., 12.41-cfs). In the site’s improved condition, larger storm
events would continue to overtop the Fossil Creek Trail; however, site improvements include a proposed 14”x 23”
replacement of the 12” culvert. A 14”x 23” culvert would be sufficiently sized to convey the improved site’s 10-year
event (i.e., 9.87-cfs).
Designed upstream of the Fossil Creek culvert is a drainage channel (Channel C), sized to convey stormwater from
Underground Ponds C1 and C2. Turf Reinforcement Matting would be included with the design of Channel C for
channel stabilization purposes. A channel capacity analysis is included in Appendix B.
Channel C would remain within the drainage easement until the landowner pursues development of the property.
In the case of future development, the landowner would potentially be required to replace the conveyance feature
with an improved feature if removed.
III.Extended Detention - WQCV
The proposed site improvements include water quality for Basins A1, A3, A6, and B1. The accumulation of these
basins necessitates a WQCV approximately 888-cf. Pond A1 is sufficiently sized to provide this combined treatment
volume with additional capacity. Below is a summary table for water quality related to Basins A1, A3, A6, and B1.
Refer to Appendix B for additional information.
Pond
Treated Area
(acre)
%
Impervious
WQCV Required
(cf)
WQCV Provided
(cf)
Pond A1 2.1 24.7 888 2,461
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Low Impact Development (LID)
In December of 2015, Fort Collins City Council adopted the revised Low Impact Development (LID) policy and criteria which
requires developments within City limits to meet certain enhanced stormwater treatment requirements in addition to more
standard treatment techniques. The proposed development will be required to meet the newly adopted LID criteria which
requires the following:
-Treat no less than 75% of any newly added impervious area using one or a combination of LID techniques.
-Treat no less than 50% of any newly added impervious area using one or a combination of LID techniques when at
least 25% of any newly added pavement is provided with permeable pavement.
The following measures are implemented with this proposed development:
I.Underground Water Quality
Basins C1-C4, combined for 164,185-sf of proposed impervious area, are designed to be routed through underground
water quality systems located in the northern region of the site. The proposed impervious area treated through
underground water quality chambers includes proposed parking, driveway, roof areas, and landscaping. The
underground water quality systems are designed with an overall combined capacity of 6,572-cf which exceeds the
minimum design requirements for the area. Refer to Appendix B for additional information.
II.Bioretention
Basins A2, A4-A5, and A7-A16, accounting for 203,061-sf of proposed impervious area, are designed to be conveyed
to Bioretention Pond A2. All new or modified impervious area related to Basins A2, A4-A5, and A7-A16 is to be
treated by Pond A2.
Basins B2-B7, accounting for 85,955-sf of proposed impervious area, are designed to be conveyed to Bioretention
Pond A3. All new or modified impervious area related to Basins B2-B7 is to be treated by Pond A3.
Provided the LID measures noted above, approximately 83% of the site is treated in its improved condition. Refer to Appendix
B for LID calculations and Appendix D for a LID Treatment Map. Below is a summary LID treatment table for reference.
75% On-Site Treatment by LID Requirement
New or Modified Impervious Area (Total) 12.20 acre
Required Minimum Impervious Area to be Treated 9.15 acre
Impervious Area Treated by LID Treatment Method #1
(Bioretention) 6.47 acre
Impervious Area Treated by LID Treatment Method #2
(Underground WQ Chambers) 3.66 acre
Total Impervious Area Treated 10.13 acre
Percent of Impervious Areas Treated by LID 83.0 %
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10 U21004_Drainage Report
Detention
I.Pond A1
The design of Pond A1 is intended for extended detention purposes for all ‘A” Basins (7.98-acres), “B” Basins (3.50-
acres), Basin C1 (0.61-acres), and Basin C2 (0.83-acres). Though Pond A1 is not physically related to Basins C1 and
C2, these basins are analyzed to provide for an area swap with Basin OS1. This detention facility is designed to release
captured runoff at a rate of 2.58-cfs in the 100-yr event, equivalent to the 0.2-cfs/acre criterion. For emergency
situations, the pond is designed to spill stormwater through the emergency overflow structure which is located at
the western end of the detention pond. Water discharged from the emergency overflow will be conveyed directly
to the Lang Gulch.
Below is a summary of Pond A1 utilizing the FAA method:
Pond Bottom of
Pond
Detention
WSEL
Top of Pond
Elevation
Detention
Volume Required
(ac-ft)
Detention
Volume Provided
(ac-ft)
100-yr
Release Rate
(cfs)
Pond A1 5002.50 5010.00 5011.00 2.85 3.05 2.58
* Refer to the provided tables in the Drainage Summary section on the following pages for context related to the
100-yr release for Pond A1.
Pond A1 is designed to release upstream of Lang Gulch avoiding the floodplain, erosion buffer and existing wetlands
near the channel bottom. Flows from the pond release will be directed to a level spreader prior to discharging onto
existing dense vegetation. A side slope analysis will be provided during final; however, due to existing vegetation
and exposed rock along the side slope, initial analysis anticipates the side slope to be stable in the proposed
conditions.
II.Pond C1
Runoff captured within Basins C3-C4 (i.e., 2.97-acres) is routed to underground chambers, designed to collect and
detain storm water runoff prior to being released to Fossil Creek. Stormwater events that exceed the water quality
event (see section Stormwater Quality narrative above) are designed to exceed the capacity of the system’s isolator
row and be further conveyed throughout the underground facility to other chambers for detention purposes. The
minimum 100-yr release via underground detention is established by a release rate per chamber to an underdrain –
this release rate is limited by flow through geotextile with accumulated sediment.
Below is a summary of Pond C1 utilizing the FAA method:
Pond
Detention Volume
Required
(ac-ft)
Detention Volume
Provided
(ac-ft)
100-yr
Release Rate
(cfs)
Pond C1 0.81 0.86 *0.59
* Refer to the provided tables in the Drainage Summary section on the following pages for context related to the
100-yr release for Pond C1.
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11 U21004_Drainage Report
Drainage Summary
A basin summary table is included below:
Existing/Historic Basins
Basin Area %I C2 C10 C100 Q2 Q10 Q100
(acre) (cfs) (cfs) (cfs)
EX1 2.12 65.0% 0.70 0.70 0.87 3.40 5.81 14.85
EX2 1.70 60.6% 0.67 0.67 0.84 2.63 4.49 11.47
EX3 0.41 82.1% 0.82 0.82 1.00 0.85 1.46 3.65
EX4 0.38 88.9% 0.89 0.89 1.00 0.96 1.65 3.80
EX5 7.97 2.0% 0.25 0.25 0.31 4.08 6.97 17.83
EX6 6.44 24.7% 0.41 0.41 0.52 5.46 9.33 23.85
OS1 3.27 43.0% 0.55 0.55 0.69 3.58 6.13 15.63
OS2 6.22 2.0% 0.25 0.25 0.31 3.19 5.44 13.91
Proposed Basins
Basin Area %I C2 C10 C100 Q2 Q10 Q100
(acre) (cfs) (cfs) (cfs)
A1 0.95 2.0% 0.25 0.25 0.31 0.51 0.86 2.21
A2 0.57 2.0% 0.25 0.25 0.31 0.30 0.52 1.32
A3 0.49 72.9% 0.76 0.76 0.95 1.06 1.82 4.64
A4 0.56 63.7% 0.69 0.69 0.86 1.03 1.76 4.48
A5 0.43 100.0% 0.95 0.95 1.00 1.09 1.85 3.98
A6 0.20 65.8% 0.71 0.71 0.88 0.38 0.65 1.66
A7 0.42 77.8% 0.79 0.79 0.99 0.95 1.63 4.16
A8 0.11 100.0% 0.95 0.95 1.00 0.30 0.51 1.09
A9 0.14 100.0% 0.95 0.95 1.00 0.39 0.66 1.43
A10 0.68 70.7% 0.74 0.74 0.93 1.34 2.29 5.84
A11 0.31 77.8% 0.79 0.79 0.99 0.66 1.12 2.86
A12 0.54 84.1% 0.84 0.84 1.00 1.14 1.94 4.74
A13 0.45 90.0% 0.95 0.95 1.00 1.15 1.97 4.23
A14 0.63 78.9% 0.80 0.80 1.00 1.20 2.06 5.26
A15 0.73 88.1% 0.86 0.86 1.00 1.51 2.57 6.08
A16 0.77 90.0% 0.95 0.95 1.00 1.95 3.33 7.16
B1 0.46 2.0% 0.25 0.25 0.31 0.25 0.42 1.07
B2 0.48 2.0% 0.25 0.25 0.31 0.26 0.44 1.12
B3 1.10 70.8% 0.74 0.74 0.93 1.80 3.09 7.88
B4 0.38 91.8% 0.89 0.89 1.00 0.75 1.29 2.95
B5 0.54 85.4% 0.85 0.85 1.00 1.10 1.87 4.53
B6 0.22 61.8% 0.68 0.68 0.85 0.40 0.69 1.76
B7 0.30 77.2% 0.79 0.79 0.98 0.68 1.16 2.97
C1 0.61 84.9% 0.84 0.84 1.00 1.37 2.34 5.68
C2 0.83 58.8% 0.66 0.66 0.82 1.46 2.50 6.37
C3 2.47 93.6% 0.90 0.90 1.00 5.13 8.77 19.83
C4 0.50 90.0% 0.95 0.95 1.00 1.27 2.17 4.66
D1 0.45 73.6% 0.76 0.76 0.95 0.97 1.66 4.25
E1 2.12 77.4% 0.79 0.79 0.99 4.22 6.80 18.41
OS1 3.27 43.0% 0.55 0.55 0.69 3.58 6.13 15.63
OS2 6.36 2.0% 0.25 0.25 0.31 3.26 5.57 14.23
P RELIMINARY D RAINAGE R EPORT
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12 U21004_Drainage Report
100-year Event: Proposed Runoff to College Ave
Basin
Designation
Flowrate
(cfs)
Basin D1 4.25
Basin E1 17.36
Proposed Runoff 21.61
Refer to Appendix D for an Existing Drainage Map that provides a visual representation of Existing Basins EX1-EX6 and OS1-
OS2. In the site’s existing condition, nearly 2.1-acres (i.e., Basin EX2 and EX4) of on-site runoff drains undetained to College
Ave. In addition to on-site basins EX2 and EX4, Basins EX1 and EX2 (i.e., College Ave) also contribute runoff to the west half
of College Ave – the entirety of Basins EX1-EX4 account for approximately 33.77-cfs in the 100-year event. In the site’s
improved condition, this 2.1-acre area is captured and conveyed to on-site detention; runoff related to all “A” basins are
detained. Moreover, the improved site is designed to convey 0.35-acres of College Ave runoff (see Basin A10). Despite the
addition of a drive lane to College Ave (i.e., additional impervious area), there is a significant reduction in runoff conveyed by
College Ave in the 100-year Event. The tables below summarize the impacts of the proposed improvements to College Ave
relative to the site’s existing condition. In the site’s its improved condition, the total runoff related to College Ave during the
100-year event is significantly reduced.
100-year Event: Existing Runoff to College Ave
In relation to the Fossil Creek, the site is designed to outfall to the west at two locations, including Lang Gulch. Ultimately, all
runoff conveyed to the west (i.e. Lang Gulch and Fossil Creek) has a similar design point in Fossil Creek, northwest of the site.
Per coordination with the City of Fort Collins Stormwater Engineering Department, swapping of basin areas is permitted given
grading constraints while maintain an overall release rate from each detention pond of 0.2 cfs/acre. The tables on the
following page summarize the impacts of the proposed improvements to Lang Gulch and Fossil Creek relative to the site’s
limiting criteria.
Basin
Designation
Flowrate
(cfs)
Basin EX1 14.85
Basin EX2 11.47
Basin EX3 3.65
Basin EX4 3.80
Existing Runoff 33.77
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Lang Gulch Outfall
100-year Event: Existing Flows to Lang Gulch
Fossil Creek Outfall
100-year Event: Existing Flows to Fossil Creek
Overall Release to Fossil Creek
100-year Event: Overall Flows to Fossil Creek
(Including upstream Lang Gulch)
Basin
Designation
Flowrate
(cfs)
Basin OS1 15.63
Basin EX6 23.85
Total Release 39.48
Basin
Designation
Flowrate
(cfs)
Basin EX5 17.83
Basin OS2 13.91
Total Flows 31.74
Basin
Designation
Flowrate
(cfs)
Basin EX5 17.83
Basin EX6 23.85
Basin OS1 15.63
Basin OS2 13.91
Total Flows 71.22
100-yr Event: Proposed Flows to Fossil Creek
Basin/Pond
Designation
Area
(acre)
Flowrate
(cfs)
Pond C1 2.97 0.59
Basin C1 0.61 5.68*
Basin C2 0.83 6.37*
Basin OS2 5.84 14.23
Total Flows 26.87
* Flowrate of Basins C1 & C2 assumes no detention. A portion of Basin OS1,
however, is detained via an area swap with Basins C1 and C2. Basins C1 & C2
are accounted for in the detention design of Pond A1.
100-yr Event: Proposed On-Site Flows to Lang Gulch
Basin/Pond
Designation
Area
(acre)
Flowrate
(cfs)
Pond A1 12.92 2.58
Basin OS1 3.70 15.63*
Total Flows 18.21
* Flowrate of Basin OS1 assumes no detention. A portion of Basin OS1, however,
is detained via an area swap with Basins C1 and C2. The remainder of the flows
from Basin OS1 will overtop the spillway of Pond A1 (approximated at 4-cfs).
100-yr Event: Overall Flows to Fossil Creek
(Including upstream Lang Gulch)
Basin/Pond
Designation
Area
(acre)
Flowrate
(cfs)
Pond A1 12.92 2.58
Pond C1 2.97 0.59
Basin C1 0.61 5.68*
Basin C2 0.83 6.37*
Basin OS1 3.70 15.63*
Basin OS2 5.84 14.23
Total Flows 45.08
* Flowrate of Basin OS1 assumes no detention. A portion of Basin OS1, however,
is detained via an area swap with Basins C1 and C2. The remainder of the flows
from Basin OS1 will overtop the spillway of Pond A1 (approximated at 4-cfs).
P RELIMINARY D RAINAGE R EPORT
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14 U21004_Drainage Report
Standard Operating Procedures (SOPs)
In order for physical stormwater Best Management Practices (BMPs) to be effective, proper maintenance is essential.
Maintenance includes both routinely scheduled activities, as well as non-routine repairs that may be required after large
storms, or as a result of other unforeseen problems. Standard Operating Procedures should clearly identify BMP maintenance
responsibility. BMP maintenance is typically the responsibility of the entity owning the BMP.
Identifying who is responsible for maintenance of BMPs and ensuring that an adequate budget is allocated for maintenance
is critical to the long-term success of BMPs. Maintenance responsibility may be assigned either publicly or privately. For this
project, the privately owned BMPs including grass swales, underground water quality and detention, and the bioretention
pond, are to be maintained by the property owner.
Storm Sewers
There are multiple storm sewer networks proposed with the site improvements. Several networks including roof drains,
storm drains, and underground water quality and detention features are proposed ultimately with outfalls to Fossil Creek.
Proposed storm sewer systems are both private and public, designed to accommodate the flows from the 100-year storm
event. Hydraulic computations of these systems will be provided with the initial final compliance submittal.
Inlets
There are multiple inlets proposed with the site improvements, including Nyloplast drain basins and Type R inlets. Type R
Inlets are designed throughout the site on-grade and in sump to capture developed runoff for detention. On-site drain basins
are designed in a sump condition and are designed to convey runoff produced within smaller interior basins to on-site
bioretention and detention facilities. All inlets on the site are sized to provide adequate capacity and convey the 100-year
storm event. Hydraulic computations of these systems will be provided with the initial final compliance submittal.
V.EROSION CONTROL
A separate Erosion Control Report / Storm Water Management Plan (SWMP) will be prepared for the site in accordance with
the Stormwater Discharge Permit for Colorado Department of Public Health and Environment as the site will disturb an area
greater than 1-acre. The Erosion Control Report will be completed during the Final Compliance phase of the project and will
include more detailed information on the sediment and erosion control items for this project. It is intended that the proposed
improvements will comply with Erosion Control Criteria per the FCSCM, and all Erosion Control Materials will be provided
with the Final Drainage Report. At a minimum, the following temporary BMP’s will be installed and maintained to control
on-site erosion and prevent sediment from traveling off-site during construction:
Silt Fence – a woven synthetic fabric that filters runoff. The silt fence is a temporary barrier that is placed at the base
of a disturbed area.
Vehicle Tracking Control – a stabilized stone pad located at points of ingress and egress on a construction site. The
stone pad is designed to reduce the amount of mud transported onto public roads by construction traffic.
Riprap – Riprap will be used downstream of all storm sewer outfalls to control erosion of the receiving channels.
Inlet Protection – acts as a sediment filter. It is a temporary BMP and requires proper installation and maintenance
to ensure their performance.
Straw Wattles – wattles act as a sediment filter in swales around inlets. They are a temporary BMP and require
proper installation and maintenance to ensure their performance.
Slope Protection – Slopes should be terraced using a “tracked” vehicle, run perpendicular to slope to inhibit rill/gulley
erosion.
The contractor shall store all construction materials and equipment and shall provide maintenance and fueling of equipment
in confined areas on-site from which runoff will be contained and filtered. Temporary Best Management Practices (BMP’s)
will be inspected by the contractor at a minimum of once every two weeks and after each significant storm event.
P RELIMINARY D RAINAGE R EPORT
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15 U21004_Drainage Report
VI.CONCLUSIONS
A.COMPLIANCE WITH STANDARDS
Storm drainage calculations have followed the guidelines provided by the Urban Storm Drainage Criteria Manuals Volumes
1, 2 and 3 and the City of Fort Collins Stormwater Criteria Manual. Moreover, Chapter 10 of the City Code has been adhered
to.
B.DRAINAGE CONCEPT
The drainage system has been designed to convey the runoff to the designated design points and the existing public
infrastructure in an effective, safe manner. No negative impacts are anticipated to the City of Fort Collins Master Drainage
Plan or to downstream properties or infrastructure due to the proposed improvements.
C.STORMWATER QUALITY
Multiple long-term stormwater quality measures have been selected for the site that will provide treatment of stormwater
prior it to being discharged from the site. For this site this includes grass swales, bioretention ponds, and underground water
quality.
VII.REFERENCES
1.City of Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, December 2018.
2.Urban Storm Drainage Criteria Manual Volume 1 and 2, Urban Drainage and Flood Control District, Denver, Colorado,
June 2001, Revised April 2008.
3.Natural Resources Conservation Service Web Soil Survey at: websoilsurvey.nrcs.usda.gov/app
4.Preliminary Flood Insurance Rate Map, FEMA, Panel 2079E, https://hazards.fema.gov/femaportal/
5.Stormwater Discharges Associated with Construction Activity, Stormwater Management Plan Preparation Guides, State
of Colorado, www.colorado.com
WATER QUALITY
Nissan Dealership, Fort Collins, CO
Required Water Quality Capture Volume
Basin Area Area Imperviousness Watershed WQCV WQ Treatment
(sf)(acres)(%)(inches)(cf)Method
A1 41,480 0.952 2%0.02 63 Extended Detention
A2 24,770 0.569 2%0.01 30 Bioretention
A3(3)21,460 0.493 73%0.29 620 Extended Detention
A4 24,280 0.557 64%0.20 484 Bioretention
A5 18,645 0.428 100%0.40 746 Bioretention
A6(3)8,780 0.202 66%0.21 181 Extended Detention
A7 18,415 0.423 78%0.25 464 Bioretention
A8 4,765 0.109 100%0.40 191 Bioretention
A9 6,250 0.143 100%0.40 250 Bioretention
A10 29,490 0.677 71%0.22 657 Bioretention
A11 13,525 0.310 78%0.25 341 Bioretention
A12 23,485 0.539 84%0.28 669 Bioretention
A13 19,780 0.454 90%0.32 635 Bioretention
A14 27,355 0.628 79%0.26 704 Bioretention
A15 31,595 0.725 88%0.31 975 Bioretention
A16 33,500 0.769 90%0.32 1,076 Bioretention
B1 20,160 0.463 2%0.01 24 Extended Detention
B2 21,090 0.484 2%0.01 26 Bioretention
B3 47,980 1.101 71%0.22 1,069 Bioretention
B4 16,646 0.382 92%0.33 555 Bioretention
B5 23,550 0.541 85%0.29 687 Bioretention
B6 9,748 0.224 62%0.19 189 Bioretention
B7 13,195 0.303 77%0.25 329 Bioretention
C1 26,565 0.610 85%0.29 768 Underground Water Quality
C2 36,362 0.835 59%0.19 675 Underground Water Quality
C3 107,575 2.470 94%0.35 3,729 Underground Water Quality
C4 21,795 0.500 90%0.32 700 Underground Water Quality
D1 19,550 0.449 74%0.29 571 None
E1 90,425 2.076 73%0.29 2,636 None
(1)Water quality provided by bioretention and based on 12-hour storage
(2)Water quality provided by underground detention and based on 12-hour storage
(3) Based on 40 hr storage
(4) Basins D1 & E1 are related to CDOT R.O.W and are not captured with the site improvemens
BMP Facility Summary
Required Proposed
Facility WQCV WQCV
Designation ac-ft ac-ft
Detention Pond A1 0.02 0.06
Bioretention Pond A2 0.17 0.29
Bioretention Pond A3 0.07 0.08
Underground Pond C1 0.04 0.04
Underground Pond C2 0.02 0.04
(2)
(2)
(3)
(3)
(1)
(1)
(1)
(1)
Date: 4/12/2022 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP
UNDERGROUND DETENTION & WATER QUALITY
Nissan Dealership, Fort Collins, CO
Required Underground Detention/WQ Volume
Chamber Total Required Detention/ Chamber Chamber Chamber *Installed Min.End Cap *Installed Min.Min. Req'd Design No. of No. of Provided Provided Total
ID Detention/WQ WQ Type Unit Unit Chamber Unit No. of Unit End Cap WQ Isolator Row Controli Chambers End Caps Release Isolator Installed
Volumea Inflowb Release Volumed Volume inc.Chambersf Volumed Volume inc. Release Volume by Provided Provided Ratej Row System
Ratec Aggregatee Aggregatee Rateg FAA Methodh Volumek Volumel
(cf)(cfs)(cfs)(cf)(cf)(sf)(cf)(cf)(cfs)(cf)(cfs)(cf)(cf)
Pond C1 (LID/WQ)4,429 3.20 MC-7200 0.043 175.90 267.30 15 39.50 115.30 0.64 1,857 WQCV 17 4 0.73 3,148 5,005
Pond C1 (Detention)35,069 24.49 MC-7200 0.043 175.90 267.30 120 39.50 115.30 ---118 26 0.6 N/A 34,539
TOTAL Pond C1 39,498 27.69 MC-7200 0.043 175.90 267.30 135 39.50 115.30 0.64 1,857 WQCV 135 30 0.60 3,148 39,545
Pond C2 (LID/WQ)1,443 1.42 MC-7200 0.043 175.90 267.30 5.00 39.50 115.30 0.21 856 WQCV 5 2 0.21 959 1,567
*Volume assumes minimum aggregate depth below chamber.
a. Total required WQCV calculated per 12-hr drain time.
b. WQ inflow is approximated as one-half the 2-yr peak runoff rate.
c. Release rate per chamber, limited by flow through geotextile with accumulated sediment.
d. Volume within chamber only, not accounting for void spaces in surrounding aggregate. The Isolator Row(s) are sized per this unit volume.
e. Volume includes chamber and void spaces (40%) in surrounding aggregate, per chamber unit. The total system WQCV is sized per this unit volume.
f. Number of chambers required to provide full WQCV within total installed system, including aggregate.
g. Release rate per chamber times number of chambers. This is used at the 'outlet control' for the FAA calculations.
h. Minimum 'chamber-only' volume to ensure dirty water is fully contained within Isolator Row.
i. Determines whether the design is controlled by the Isolator Row volume or WQCV.
j. Release rate per chamber times number of chambers provided.
k. Volume provided in chambers only (no aggregate storage). This number must meet or exceed the required FAA storage volume.
l. System volume includes total number of chambers, plus surrounding aggregate. This number must meet or exceed the required 12-hr WQCV.
Date: 4/8/2022 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP